484 CHEMICAL ENGINEERING
Similar to the change in volume the porosities of a material fired to different temperatures may be plotted against the temperature. The curves thus obtained are equally instructive and valuable, for the comparison of the pyro-technical properties of refractories. The lower curve of Fig. 3 gives the results obtained with a fireclay material. Here again the rate of porosity decrease, the temperature at which the structure has become dense, shown by the approach to zero porosity, and the evidence or overfiring offer data of practical importance. The accuracy of the results obtained depends of course upon the accuracy of the temperature measurements. Too much attention cannot be given to the calibration of the pyrometers.
Since the determination of the porosity depends to a large extent upon the absorption of water by the material it will suffice in many cases to make use only of this value in establishing trie relation between temperature and its effect upon the consolidation or vitrification of the body. In determining the absorption we need to know only the weights of the dry specimen and its weight when saturated with water, and the computation consists of the evident relation 100 (w d}/d, where w = weight of saturated specimen, and d = weight of dry test piece. The value merely expresses the weight of water absorbed in terms of the dry weight of the specimen. The porosity value is to be preferred in all cases where closer differentiation is desirable or where the comparison of bodies possessing different specific gravities is involved.
Occasionally it may be desirable to determine the total porosity of refractories, especially where they are dense and the absorption of water is incomplete or where enclosed pores exist into which the liquid cannot penetrate. For the calculation of this value it is necessary to determine the bulk and the true specific gravity. The former is determined by obtaining the weight of the dried piece, in air, and its exterior volume, as described in a preceding paragraph. The bulk density obviously is equivalent to the relation w/v, where w "weight of specimen and v = its volume. The true specific gravity is found by crushing the material, passing it through the 100-mesh sieve and determining the values sought by means of the pycnometer in the usual manner. The true porosity is then computed from the relation: p = 100(1 di)/dz, where p = per cent pore space in terms of the exterior volume, d = true specific gravity and ^2 = bulk specific gravity.
Specific Gravity.It is interesting to note that most silicates upon being heated increase in specific volume, that is, decrease in density, in spite of the fact that the exterior volume of the piece may contract. It is evident that this phenomenon is to be ascribed to molecular changes and to the progress of softening or fusion. In general, the porosity-temperature curves parallel the specific gravity-temperature plots. Fireclays higher in fluxing impurities invariably show a lower specific gravity at the same temperature than purer materials, but even refractory clays exhibit an appreciable drop in density. Thus a high-grade Kentucky flint clay of specific gravity 2.60, when fired to 1,550°C., possessed a density of 2.38. From the technical standpoint the determination of the specific gravity is often of service in quickly estimating the degree of firing to which a refractory has been subjected for the purpose of judging whether it has been brought to the proper end point, especially in the case of silica and magnesite brick.
The true specific gravity is also of great value in theoretical studies for the observation of transformation points. Since fusion from the exact standpoint represents a change of state which may be located by the discontinuity of some physical property, it may represent the intersection of the vapor-tension curves of the crystalline and